The objective of our proposed research is to determine at the molecular level the details of the mechanism by which mammalian cells bypass ultraviolet light induced damage in their DNA during DNA replication. A further objective of this study will be to investigate whether this repair process is applicable to other types of damage such as lesions produced by ionizing radiation and mutagenic and/or carcinogenic chemicals. Techniques to be used include: paper and column chromatography, to assay for dimer excision, alkaline sucrose gradient to detect breaks in DNA and then rejoining, CsCl gradients to measure extent of DNA replication. Cell sensitivity and recovery of colony forming ability between fractionated doses are used as indicators at the cellular level of repair which is active at the molecular level. To the extent that it is possible, we try to use synchronized populations for our experiments. In this way we can determine the changes in cell sensitivity and events at the molecular level as a function of the cell cycle.